1. Field of the Invention
This invention relates to a main-shaft malfunction-state detector in an air bearing type machine tool, using for detecting an abnormal contact of a main shaft in a machine tool for a cutting process of a relatively heavy load in which the main shaft is supported in an air static-pressure bearing, and so on.
2. Description of the Related Art
Conventionally, a machine tool is used for, for example, cutting various metallic materials.
The machine tool has a cutting tool attached at an end of a main shaft rotatably supported, in which the tool is rotated by rotatably driving the main shaft with a shaft driving motor, thereby performing a cutting process for a work piece.
Support for the main shaft
The conventional machine tool uses various bearings for rotatably supporting the main shaft.
For example, an air static-pressure bearing is used for the machine tool for carrying out an ultra-precise cutting process for the cutting light load (e.g., in the case of a mirror finish of light-metal material by using diamond bit).
In the aforementioned process, it has been difficult to produce deformation of the main shaft because of the light load, and there is little possibility that the main shaft will be in contact with the housing.
As to the air bearing
The air bearing is, conventionally, used for only process for the light load, however, the present applicant proposes the air bearing type machine tool which is capable of being used for the heavy load by enlarging a bearing clearance (see Japanese Patent Application Laid-open No. Hei 8-70219).
Here, where the air bearing type machine tool is used in the process for the high load, the deformation of the main shaft caused by the load occurs easily, so that attention to the abnormal contact of the main shaft is required.
Further, in the normal cutting process, there is the potential that the process is carried out under conditions exceeding the forecast made in the design.
For example, in the process where the tool attached at the main shaft is projected more than usual, a large moment is acted to the bearing portion. Generally, in the air bearing, the main shaft should not be in contact with the bearing portion of the housing, but an abnormal contact is produced when the degree of the deformation of the main shaft, caused by the moment, exceeds the space between the bearing and the main shaft.
If the operation continues while the abnormal contact of the air bearing described above remains, the disadvantage is that a seizure is produced resulting in a break.
In order to avoid the break, it is required the main shaft is immediately stopped to stop the process when the aforementioned abnormal contact is produced.
In order to avoid breaking, it is required that the process is immediately stopped by stopping the main shaft when the aforementioned abnormal contact is produced. The detection of the abnormal contact, conventionally, depends on the alertness of an operator, resulting in disadvantages concerning operating efficiencies, a delay in the detection, and so on.
It is an object of the present invention to provide the main-shaft malfunction-state detector for the air bearing type machine tool, in which an abnormal contact of the main shaft with the housing is reliably and automatically detected.